This invention relates to cathode ray tubes (CRTs) for color television, and more particularly relates to the bimetal springs which support the aperture mask-frame assembly in the face panel portion of the glass envelope of the CRT.
Cathode ray tubes (CRTs) used in color television commonly employ an aperture mask suspended a short distance behind the face panel portion of the tube's glass envelope. The function of the aperture mask is to direct the electron beams from the electron guns in the neck portion of the envelope to the proper phosphor elements of the luminescent display screen, which is located on the inner surface of the face panel. The suspension arrangement must be capable of maintaining the mask in the proper position during the tube's operation, in order to prevent mis-registration of the apertures and the phosphor elements. Such mis-registration could result in the electron beams landing on the wrong phosphor elements, causing degradation of the color purity of the display image.
In one design, the mask, which is a relatively thin sheet of metal containing a large number of small apertures, is supported by a heavier gage metal frame, and the frame is supported in the panel by three or four bimetal springs mounted to the sides of the frame. These springs are designed to deflect upon heating during the tube's initial warm-up period in order to maintain registration between the mask and screen.
One end of each support spring is welded to the mask frame, while the other end defines an aperture which engages a mounting stud embedded in the side wall or skirt of the glass face panel. Between the ends, the spring is bent in order to lift the main portion of the spring out of the plane of the welded end. The angle of the bend determines the spring load required to deflect the spring to allow insertion of the mask-frame assembly into the panel.
Because registration between the mask apertures and the phosphor elements on the luminescent screen is so critical, the mask is used in the manufacture of the screen. In order to insure registration, the photosensitive layers containing the different color phosphors are exposed through the mask. Because there are three separate exposures for three separate colors (red, blue, green), the mask-panel assembly must be inserted and removed from the panel several times before final assembly.
After final insertion of the mask-frame assembly into the panel to form the panel-mask assembly (PMA), it is critical to the operation of the tube that the registration between the apertures and the phosphor elements be maintained. If the picture tube is mis-handled, for example, dropped or jarred during packing, shipping or storing, the mechanical forces on the tube will be transmitted to the support springs through the panel via the studs. Since the springs are cantilevered, they can experience very high torsional loading in the area of the bend radius. If the torsional loading causes permanent deformation of one or more of the springs, the tube must be rejected.
Various means have been suggested to improve the resistance of such support springs to mechanical shock. In JP-A 56-97944, the main portion of the spring is constituted by two slightly non-parallel legs; in JP-A 56-97945, the main portion of the spring is constituted by two legs, one of which has an arc shaped curve; other patents, such as U.S. Pat. Nos. 3,986,072; 4,663,561; and 4,670,687, describe mask assemblies which do not have rigid support frames and which have corner mounting systems employing leaf springs having lower spring rates than the side-mounted springs employed with rigid frames.